FIG. 1 is a simplified block diagram of a synthetic aperture radar (SAR) system synthesized from the prior art. In FIG. 1, a vehicle (not illustrated) such as an airplane bears a fan-beam transmit-receive antenna 10 which produces a footprint on the ground being imaged. Antenna 10 is coupled to a transmit-receive (TR) device 12, which allows the same antenna to be used for both transmission and reception. TR device 12 is in turn coupled to a transmitter 14 and to a receiver 16. Transmitter 14 produces transmit pulses at a rate and with a duration controlled by a timing generator or pulse source 18, applied over a path 19a. Such pulses typically have a pulse repetition rate in the kilohertz or tens of kilohertz range, and a duty cycle in the vicinity of 50%, but other characteristics may be used. The transmit pulses are illustrated as a pulse train 210 in FIG. 2a. The pulses from transmitter 14 are coupled through TR device 12, and are transmitted by antenna 12 toward the ground. The pulse energy arriving at the ground has an elliptical footprint, as suggested by 310 of FIG. 3. In FIG. 3, the vehicle carrying the SAR antenna, transmitter and receiver is illustrated as a block 312, moving in the direction of arrow 314 with velocity v. The altitude of the aircraft is designated h, and the distance from the sub-aircraft point to the target region is designated X.sub.0. The range is readily determined, and is illustrated as R.sub.0. Those skilled in the art realize that the footprint 310 moves with time as the vehicle carrying the radar system moves.
The energy which falls on the footprint of FIG. 3 is reflected, depending upon the characteristics of the particular portions of the terrain and any targets thereon. A portion of the reflected energy is returned to the antenna in the form of complicated modulation of the original pulse, and is illustrated as 212 in FIG. 2b. The modulated pulse information returned to antenna 10 of FIG. 1 is coupled by TR device 12 to receiver 16, which performs analog signal processing as may be required, such as low-noise amplification, frequency conversion, and the like, and couples the resulting received signal to a digitizer, illustrated as 20. Digitizer 20 converts the received signal to digital form, and applies the resulting digital received signal to a SAR processor illustrated as a dash block 22. SAR processor 22 performs various types of processing for converting the received signals into a representation of the region being imaged by the SAR radar system, and couples those image-representative signals to an image processor illustrated as a block 24, which adjusts the signals for best display, as for example by allowing selection of appropriate contrast, colors, or the like, or for actual processing such as for correlation of the current image with a stored image.
Within SAR processor 22 of FIG. 1, the digitized signals are first applied to a first processor (PROC) 30, which is used when high-resolution images are to be generated. High resolution is achieved by breaking the received signal samples into sub-sets, and further processing the subsets to generate multiple sets of image data, from which fine detail can be extracted. One simple way to divide the signal samples into different sets is to multiplex the received signals sequentially, in the order of reception, to a plurality of channels, illustrated as channel 1 (C1) . . . channel N (CN) in FIG. 1. Each of the sets of samples traverses its own SAR processor 34a, . . . 34N, and the resulting images are recombined by a second processor 32. The high-resolution image-representative data at the output of processor 22 is coupled to image processor 24, for further adjustment. In the event that less resolution is needed, a single SAR processor, such as SAR processor 34a, may be used, in which case processors 30 and 32, and the other illustrated SAR processors 34, may be dispensed with.
High-resolution SAR images are not now generated aboard the aircraft which does the SAR sensing. Instead, the digitized information from digitizer 20 of FIG. 1 is transmitted from the sensing aircraft to a ground station, and the computations are made by ground-based computers. It would be desirable to be able to include the computers in the aircraft, so that the images could be viewed directly as the signals are received, and to obviate the need for high-bandwidth communications from aircraft to ground, but the required computations are sufficiently complex so that ground-based computers are more advantageous.
FIG. 4 is a simplified block diagram of an optical SAR processor. In FIG. 4, a spatial light modulator (SLM), described in greater detail in FIG. 5, receives the digitized return signals from digitizer 20 of FIG. 1, and produces a diffraction pattern as described below, which propagates across the length of the SLM. The dimensions of the SLM are selected so that the signals applied to one end of the SLM propagate to the other end of the SLM in one period of the transmitted pulses 210 of FIG. 2a. That is, the returned signal resulting from each transmitted pulse exactly fills the length of the SLM. Naturally, some margin or leeway must be allowed for SLM control time and for errors. An example of the pattern of refraction across the surface of SLM 410 resulting from one transmitted pulse is illustrated as 412 in FIG. 4.
Spatial light modulator 410 of FIG. 4 is reflective, as described below. A source of light such as a laser 414 in FIG. 4 produces a beam 416 of coherent light, which is processed by an optical system 418, if necessary, for expanding the beam to the dimensions of the active portion of spatial light modulator 410. The expanded beam 416 of light is reflected and directed toward spatial light modulator 410, where it is modulated by the pattern of refraction then existing or propagating on its surface due to the received signal. The modulated beam reflected or diffracted from the surface of SLM 410 is applied to half-silvered mirror 426, and passes through the mirror to a Fourier transform lens 428. Lens 428 performs a Fourier transformation to produce a Fourier-transformed image at the F-T plane 430. A mask, illustrated in exemplary form as 432, lies in the FT plane 430, and allows only a portion of the transformed light portion to pass. In effect, mask 432 represents a spatial filter in the Fourier plane. The filtered , transformed light exiting mask 432 is applied to an imaging lens, which images the Fourier plane onto the surface of a detector 436, which may be an array of photosensors, or preferably a charge-coupled imager device (CCD) or camera. An interference pattern is formed at the surface of detector 436 by combining the light from imaging lens 434 with unmodulated light from laser 414. The combining is accomplished by a mirror 438 and a light combining half-silvered splitter 440, which couple light beam 422 to the surface of light sensor array 436. The purpose of the interfering light is to allow phase information to be recovered, which provides an improved image. An electrical signal representing the image applied to detector array 436 is generated on an output signal path 442. As so far described, the arrangement of FIG. 4 includes a single channel, which recovers amplitude or phase information from the received signals, but not both. In the arrangement of FIG. 4, mask 432 is selected, in known fashion, to recover amplitude information. A second channel is provided by a light splitter 450 and a mirror 452, which couple a further beam 454 to a second Fourier transform lens 456. Lens 456 performs a Fourier transform to a Fourier plane 458. A second mask or filter (not illustrated), which is similar to, but not identical with, mask 432, is placed at the second Fourier plane 458, to extract phase information from the received signal. The phase mask filters the Fourier-transformed light, and the resulting image is focussed onto a second light detector array 460 by an imaging lens 462. An interference is provided by a sample of the unmodulated coherent light coupled to the surface of detector 460 by a splitter 464. The arrangement of FIG. 4 with the inclusion of the second channel is desirable when only a single SAR processing channel is to be used.
FIG. 5 is a simplified representation of an acoustic light modulator which may be used in the arrangement of FIG. 4. In FIG. 5, modulator 410 is a lithium niobate crystal 512 with an electrical-to-acoustic transducer 510 at one end thereof. Transducer 510 includes a pair of electrodes to which the received signal is applied, to create an acoustic wave which propagates across the surface of the crystal in the direction of arrow 514. The acoustic surface wave causes changes in the index of refraction, illustrated as a pattern of transverse lines 516, which propagate across the surface of the crystal. Ideally, the acoustic waves are dissipated upon reaching the end of the crystal, so as not to reflect and interact with the pattern of the index of refraction across the surface. The pattern of index of refraction represents the amplitude of the received signal for a period of time. Unfortunately, the acoustic modulator is subject to a number of imperfections, including "spreading" of the surface wave into the bulk crystal, as suggested by the "frequency and spatial dispersion" arrow 518, and reflections from the end of the crystal. The length of the crystal is selected in conjunction with the pulse repetition rate of the transmitted pulses, so that the time required for an acoustic wave to propagate across the crystal is equal to the duration of a transmitted pulse. Thus, as the acoustic wave propagates across the surface of modulator 410 of FIG. 5, there is an instant at which the entire surface of the crystal is "filled" with a pattern of indices of refraction which arise from the returned signals caused by one transmitted pulse. At that instant, laser light 414 of FIG. 4 is pulsed ON, to illuminate the surface of modulator 410, and to thereby generate the modulated light pattern required for the desired Fourier processing, illustrated in FIG. 5 by arrows 522, representing different targets.
The frequency content of the returned signals may extend above the tens of megahertz, and even into the hundreds of megahertz. The ability to process signals with such a bandwidth is one of the advantages of the optical signal processing described in conjunction with FIG. 4. As a result of the very large bandwidth of the processing, the shortest-duration pieces of information existing on the surface of modulator 410 of FIG. 5 are a fraction of the duration of the highest frequency being processed. If the highest frequency being processed is 100 MHz., the duration of one cycle at the highest frequency is ten nanoseconds, and the shortest-duration sample must be no longer than five nanoseconds. Thus, in order to fully process the returned signal appearing on the surface of acoustic modulator 410 of FIG. 5, the duration of the light pulse must not exceed five nanoseconds for a useful bandwidth of 100 MHz. Of course, the actual duration of the pulse of light depends upon the bandwidth. A coherent light pulse of such short duration is difficult to produce, requiring very specialized laser sources, and also difficult to process, because of the losses inherent in the optical components, which reduces the amount of light available at the detectors 436 and 460 of FIG. 4. Unfortunately, it is not possible to keep the laser light source 414 ON for a longer period of time than that allowed by the bandwidth limitation mentioned above in order to allow more photons to be integrated by each photosensor of the detectors, because longer-duration light pulses allow the pattern on the surface of the light modulator to change during the pulse. These changes occur because the acoustic waves continue to propagate during illumination, with the result that the image becomes "blurred". When the pulses of light are very short, the resolution is high, but the small amount of light results in poor signal-to-noise ratio (SNR), and when the pulses of light are longer, the signal-to-noise is improved, but the image resolution suffers. Improved SAR processors are desired.